Deinstall it, go to C:\Users\your user name\AppData\Roaming\Bitcoin and delete everything. Make a backup of your wallet.dat before that if you have used it.
Next download Electrum or Mutibit from
http://bitcoin.org/en/choose-your-wallet and not the BitcoinQT client. As Bitcoin.org states:
Bitcoin-Qt is the original Bitcoin client and it builds the backbone of the network. It offers the highest levels of security, privacy and stability. However it has fewer features and it takes a lot of space and memory.
It's still something weird.
I did as you said and when i checked i had some GB's left and now i checked again a couple of minutes later it's lesser, therefore it has changed without me doing anything..